1. Field of the Invention
The present invention relates to a large deformation technique for metal-based materials, and more particularly to a large deformation apparatus for reducing the crystal grain size of plastically deformable materials, and preferably metal-based materials and metal-based composite materials, by continuously subjecting the materials to large deformation without removing these materials from the mold; to a deformation method therefor; and to a material which is subjected to such continuous large deformation and in which the crystal particles of the matrix are reduced to a grain size of 10 .mu.m or less.
2. Description of the Related Art
It is generally well known that reducing the crystal grain size of a polycrystalline material is effective for improving the strength and ductility of this material. In conventional practice, therefore, the crystal grains of plastically deformable materials typified by metal-based materials are destructed and recrystallized to achieve a smaller crystal grain size by performing plastic working based on extrusion or rolling at a high temperature above the recrystallization temperature. The work materials are limited in their post-work shape to a wire-rod shape in the case of extrusion, and to a thin-sheet shape in the case of rolling, and these shape limitations impose restrictions on the post-work applications of these materials.
By contrast, Equal-Channel Angular Pressing (ECPA) is a method in which a work material is subjected to shear deformation at a temperature below the melting point of the material by being passed through a curved hole obtained by curving the middle portion of a through hole at a given angle. In this work method, the material can be subjected to large plastic deformation with minimal changes in the external shape of the material before and after working, making it possible to reduce the size of the crystals constituting the work material. An example of this method is the process described in the report by Horita et al. (Materia Japan, Vol. 37, 767-774 (1998)), particularly one shown in the appended drawings.
As described in detail with reference to the aforementioned drawings, this work method is one in which the work material is passed through a curved hole, but a single passage is insufficient for reducing the size of the crystals constituting the material, so large deformation must be repeated at least several times, and usually ten or more times. In other words, the work material usually must be passed through the curved hole after being heated to the working temperature. Consequently, the work material must be repeatedly taken out of the mold outlet and inserted into the mold inlet after passing through the curved hole, and hence must be heated to the working temperature after being inserted into the mold because the temperature of the work material inevitably decreases when the material is taken out of the mold.
A resulting drawback is that complicated procedures must be performed to control the temperature of the work material and that thermal energy commensurate with the reduction in the temperature of the work material must be provided for each work cycle, resulting in a process that is economically disadvantageous and that is time-consuming and inefficient because of the need to wait for the temperature to reach the working level. In addition, the work material is exposed to the atmosphere, undergoing oxidation (which depends on the composition of the material) and creating a burn hazard for the workers.
An urgent need therefore existed for an apparatus and method that would allow a work material retained inside a mold provided with a curved hole to be continuously subjected to the aforementioned high plastic deformation without being taken out of the mold to repeatedly perform the aforementioned high plastic deformation.
According to another method of applying large deformation, materials are shaped as wire rods or thin pieces by being repeatedly inserted into and taken out of variable-diameter continuous holes in accordance with mechanical alloying techniques (Aizawa et al., Kinzoku (Metal), Vol. 65 (1995), 1155-1161). Since mechanical alloying involves processing powder samples, not only it is different from the large deformation method of the present invention in its nature, but there is a risk that cracks will form on the surface of the material as it moves from a smaller hole to a larger hole, and because only a small amount of processing energy is applied to the unprocessed material, several hundred work cycles (depending on the material) need to be performed, resulting in an extremely time-consuming and inefficient process.
According to another method, a material is subjected to large deformation by being alternately pushed in and drawn in the vertical and horizontal directions (Fujita et al., Kinzoku (Metal), Vol. 65 (1995), 1143-1154), but this method is similar to the above-described Aizawa technique in that it involves performing mechanical alloying. In addition, this method is completely unsuitable for processing bulk materials because it necessitates splitting the work material in two in the axial direction. This method thus cannot be used as a means for solving the above-described problems, and an urgent need for finding such a means still remains.
Studies have been conducted concerning the extent of large deformation in work materials during their ECPA processing in holes having bending angles of about 120 degrees and 90 degrees, and it was found that an angle of 90 degrees provides greater deformation.
With the foregoing in view and as a result of repeated and painstaking research conducted with consideration for the above-described prior art and aimed at developing a method for applying large deformation and continuously working a material in a mold without taking this material out of the mold, the inventors perfected the present invention upon discovering that using an apparatus configured as described below allows large deformation to be continuously applied to a material without reintroducing the material into the mold.
An object of the present invention is to provide a large deformation apparatus for a metal-based material that allows materials subjected to large deformation to be continuously subjected to large deformation inside a mold without being taken out of the mold; to provide a work method therefor; and to provide a material whose crystal grains can be reduced in size by the application of such large deformation.